Conventional particle beam microscopes use electron beams, proton beams, ion beams, x-ray beams and others as the particle beams. In some particle beam microscopes, the particle beams are finely focused at a location in the sample. In such microscopes, the location at which the particle beam is focused can be displaced relative to a main particle beam direction in an axial direction as well as a lateral direction to be able to scan the beam focus across the sample. Detection signals of particles having interacted with the sample may be recorded in dependence of the location at which the beam is focused in order to obtain a two-dimensional image or a three-dimensional volume image of the sample. In such situations, an extension of the particle beam focus determines a spatial resolution of the microscope. In view of a high lateral resolution, the focus region should have a small lateral extension, and in view of a higher longitudinal resolution, the focus region should have a small longitudinal extension.
The particles of the focused particle beam are incident on the sample from different directions. Generally, the different directions are from within a cone having apex angles +α and −α relative to a main beam direction such that the total apex angle is 2α. The longitudinal resolution is related to a depth of focus of optics used for focusing the particle beam, wherein the depth of focus is proportional to 1/α2. In view of a high longitudinal resolution, the apex angle of the cone should have a high value. However, particle optical lenses used for focusing the particle beam typically exhibit an opening error which is proportional to α3. High values of the apex angle result in a relatively low lateral resolution, accordingly.
An example of a particle beam microscope is a scanning transmission electron microscope as disclosed in U.S. Pat. No. 6,548,810 B2, the entire contents of which are hereby incorporated by reference.